Junmei Yao

Symbol-level detection: A new approach to silencing hidden terminals

Hidden terminals are typical interference sources that can significantly reduce the throughput of a wireless network if it adopts the CSMA/CA MAC protocol. The RTS/CTS mechanism is a well-known solution to this hidden terminal problem. However, it only works well under the assumption that all hidden terminals can decode the CTS packets correctly. In the real world, the CTS packets might not be correctly received all the time due to either the CTS packets are unable to be decoded at remote hidden terminals or the CTS packets are collided with other packets at the hidden terminals. Both of these drawbacks can make the standard RTS/CTS mechanism fail to silence all hidden terminals, and deteriorate the throughput of the wireless network. In this paper, we present the RTS/S-CTS mechanism, a novel symbol-level detection mechanism that combats these two drawbacks. The RTS/S-CTS frames make slight changes to the standard RTS/CTS frames, and can be compatible with the standard 802.11 MAC layer. We design the symbol-level detection decoder (SLDD) and NAV decision algorithm that enable the S-CTS frame to be correctly detected from collisions and by remote hidden terminals. We build a testbed of RTS/S-CTS with GNURadio/USRP2 software radio to demonstrate its feasibility and run ns-2 simulations to evaluate its performance. The results show that the RTS/S-CTS can achieve up to 63% throughput improvement in the random topology network scenario compared with the standard RTS/CTS.

Beyond the Limit: A Fast Tag Identification Protocol for RFID Systems

This paper presents the design, experiment test and performance evaluation of FAST, a fast tag identification protocol in RFID systems. FAST includes a collision tolerant mechanism (CTM) that can identify tags in collision slots correctly. Each tag sends a randomly selected signature in its selected slot. When multiple tags select the same slot and a collision occurs, the reader can detect all the signatures correctly by exploiting the cross correlation. It then asks tags to send their tag IDs sequentially through a series of ACK commands, leading to no collision in the tag ID transmissions. By using CTM, all the tags in the collision slots can be identified successfully if the number of tags involved conforms to the signature detection limit. FAST also includes a dynamic frame length estimation mechanism to maximize the slot utilization so as to further improve the performance of tag identification. Theoretical analysis and experimental results from the USRP2 testbed demonstrate the feasibility of FAST. Simulation results show that FAST outperforms other protocols dramatically.

On Demand Response Management Performance Optimization for Microgrids Under Imperfect Communication Constraints

A perfect bidirectional communication network is a common assumption in smart grids. However, it is unrealistic, especially in the neighborhood area network of microgrids. Due to the channel fading, large volumes of transmission data, and considerable communication cost, the imperfect communications affect the system performance directly. In this paper, we consider the uncertainty of imperfect communications in both supply and demand sides, which affects the microgrid system performance in terms of the packet loss ratio of the power demand data transmission and the forecasting accuracy ratio of the renewable energy generation. We analyze the impacts of imperfect communications on the demand response management (DRM) performance under the real-time pricing scheme. An optimization problem is formulated first to maximize the DRM performance of the microgrid system. As these impacts can be mitigated by using sufficient spectrum resources, we then propose a spectrum resource allocation scheme that considers different characteristics of transmission data and system communication cost to balance the tradeoff between the DRM performance and the incurred communication cost. We introduce a joint optimization problem that not only maximizes the DRM performance but also minimizes the communication cost. Simulation results reveal the impacts of imperfect communications on the DRM performance and power price, and the efficiency of the proposed optimization problems.

It Can Drain Out Your Energy: An Energy-Saving Mechanism Against Packet Overhearing in High Traffic Wireless LANs

Energy efficiency is a critical issue of wireless devices. As the packets are broadcast to the devices in the wireless transmission media, all active neighboring devices have to spend their energy receiving the packets though the packets are not addressed to them, which is called as the packet overhearing problem. The real-world traffic trace analysis reveals that the energy cost on the packet overhearing accounts for the majority of the devices’ energy inefficiency in high traffic wireless local area networks (WLANs). In this paper, we propose a novel sample-address sample-duration (SASD) scheme to solve the energy inefficiency of the packet overhearing problem. By adding a new SASD header, which contains the critical information, in front of the data packet at the physical layer, the SASD enables the devices to discern the required information in the energy-saving downclocking mode. Consequently, the non-destination devices of the packet can switch to the sleeping mode to avoid the packet overhearing problem. We demonstrate the feasibility of the SASD through hardware experiments and evaluate its energy-saving performance through ns-2 simulations. The results show that the SASD can greatly outperform the existing approaches in the high traffic WLAN scenario.

Revisiting of Channel Access Mechanisms in Mobile Wireless Networks through Exploiting Physical Layer Technologies

The wireless local area networks (WLANs) have been widely deployed with the rapid development of mobile devices and have further been brought into new applications with infrastructure mobility due to the growth of unmanned aerial vehicles (UAVs). However, the WLANs still face persistent challenge on increasing the network throughput to meet the customer’s requirement and fight against the node mobility. Interference is a well-known issue that would degrade the network performance due to the broadcast characteristics of the wireless signals. Moreover, with infrastructure mobility, the interference becomes the key obstacle in pursuing the channel capacity. Legacy interference management mechanism through the channel access control in the MAC layer design of the 802.11 standard has some well-known drawbacks, such as exposed and hidden terminal problems, inefficient rate adaptation, and retransmission schemes, making the efficient interference management an everlasting research topic over the years. Recently, interference management through exploiting physical layer mechanisms has attracted much research interest and has been proven to be a promising way to improve the network throughput, especially under the infrastructure mobility scenarios which provides more indicators for node dynamics. In this paper, we introduce a series of representative physical layer techniques and analyze how they are exploited for interference management to improve the network performance. We also provide some discussions about the research challenges and give potential future research topics in this area.

On Charging Scheduling Optimization for a Wirelessly Charged Electric Bus System

The introduction of wirelessly charged electric buses (WCEBs) into current public transportation system attracts many attentions in recent years. As the wireless charging technology enables energy transfer from power transmitters to electric vehicles (EVs) on road, it provides a promising solution to reduce the huge cost of battery with large size and long charging time, which are two critical impediments for EV applications. However, the system cost of WCEBs is huge. Under the dynamic electricity demands and the fluctuating electricity prices, the system operating electricity cost highly depends on the charging schedule. In this paper, according to the typical day-ahead electricity market, we explore an optimal charging scheduling scheme in a WCEB system to minimize the system operating electricity cost, while the characteristic of WCEBs is considered. The price of electricity fluctuates with the accumulated energy demands in both spatial and temporal domains. We first present a day-ahead reserved wholesale electricity determination algorithm, in which, the average speeds of WCEBs are presumed. Then, we propose an optimal charging scheduling algorithm, in which the WCEB charging schedules in slots are optimized sequentially. Both the reserved electricity and the predicted speeds in the slot are used. Simulation results demonstrate the efficiency of our proposed WCEB charging schedules.

Efficient Interference-Aware Power Control for Wireless Networks

Abstract Interference management through power control in wireless networks has both the hidden terminal problem which induces transmission collisions, and the exposed terminal problem which blocks concurrent transmissions. Both problems are caused by the varied interference range of different links. Through observing that the nodes adopt the power control mechanism induce collisions in one scenario and miss concurrent transmission opportunities in two scenarios, this paper presents IAPC (Interference-Aware Power Control), a novel protocol to improve the network throughput from the three aspects. IAPC makes the interference range of each link covered by its CTS (Clear-To-Send) transmission through utilizing a signature detection process, so as to avoid interference. Meanwhile, it makes the CTS frame carry the transmission and reception power information of this link, according to which the neighboring node can determine a limited transmission power, trying to make itself outside the interference range of the ongoing link, so as to exploit concurrent transmissions. The signature detection range has been investigated through both experiment results and theoretical analysis, and the performance improvement of IAPC comparing with the other protocols has been shown through simulation results based on ns2.

Exploit concurrent transmissions through discernible interference cancellation

This paper represents the design, feasibility evaluation and performance validation of ICMR, a novel cross layer protocol that can maximize concurrent transmissions and avoid data frame interference in wireless networks, achieving higher throughput comparing with the 802.11 standard and other state-of-the-art protocols. Observations on the 802.11 standard reveal that nodes degrade the network throughput from two aspects, including the so-called CF-CA problem and varied-IR problem, and these problems will make nodes around both the transmitter and receiver of the ongoing link waste concurrent transmission opportunities. A state-of-the-art protocol IRMA is proposed to improve the network throughput through solving the two problems at the transmitter side. In this paper, a new ICMR protocol is proposed to solve both problems at the receiver side to further improve the network throughput through discernible interference cancellation, a physical layer mechanism that can successfully detect data frames when collided by control frames. Hardware experiments based on USRP2 demonstrate the feasibility of the discernible interference cancellation mechanism, and simulations based on ns-2 confirm that ICMR outperforms the 802.11 standard and other protocols significantly.

Efficient interference-aware power control in wireless ad hoc networks

Interference management through power control in wireless ad hoc networks has both the hidden terminal problem which induces collisions, and the exposed terminal problem which prohibits concurrent transmissions. Both problems are caused by the varied interference range induced by the adjusted transmission power. Through observing that the nodes adopt the power control mechanism induces collisions in one scenario and miss concurrent transmission opportunities in two scenarios, this paper presents IAPC (Interference-Aware Power Control), a novel protocol to improve the network throughput from these aspects. IAPC makes the interference range of each link covered by its CTS (Clear-To-Send) transmission through utilizing a signature detection process, so as to avoid interference. Meanwhile, it lets CTS convey the transmission power information of this link to make the neighboring node determine a limited transmission power, which tries to make the neighboring node outside the interference range of the ongoing link, thus can exploit concurrent transmissions. Simulation results based on ns-2 show that IAPC can outperform the other protocols significantly.

Coordinate Transmissions Centrally: A Cross-Layer Approach for WLANs

This paper represents the design, feasibility evaluation and performance validation of concurrency-based coordination mechanism (CCM), a novel cross-layer protocol that can coordinate among nodes effectively to avoid data packet interference in wireless local area networks (WLANs), achieving higher throughput compared to 802.11 standard and other state-of-the-art protocols. The design of CCM contains OpenCCM which is based on the architecture of software defined network to schedule the transmissions in both the uplink and downlink directions centrally to maximize transmission concurrency. It also contains an interference-resistant mechanism in the physical layer that can make the control message transmitted with the data packet simultaneously to eliminate the coordination overhead. Experiment results with USRP2 demonstrate the feasibility of the interference-resistant mechanism, and the simulations by ns-2 show that CCM can outperform other protocols significantly.